The present invention relates to a device for feeding and transporting printing forms which can be transported onto an exposure table with the aid of rollers. A device of this type has rollers which are located at the two lateral edges of the printing form, and those rollers positioned first, in the transport direction of the printing forms, can be pivoted outwardly.
The production of printing forms carrying lettering and images, for example printing plates which are immediately suitable for printing, is carried out by initially charging the printing plates electrostatically and then projecting an original, as an image, onto the individual printing plate. Following termination of this exposure process, the printing plate is developed with a developer, fixed and decoated, after which the plate is ready for the printing operation.
A unit for the electro-photographic production of printing forms is known from U.S. Pat. No. 4,006,984. In this unit the individual printing form is lifted from a stack in a plate magazine by means of a transport device which transports the printing forms to an exposure stage, wherein the transport device comprises a transport carriage having a suction arrangement operated by a pressure lower than atmospheric pressure. In order to combine the charging-up of the individual printing form with its transport to the exposure stage, so as to save time, a corona-type charging station for electrostatically charging the printing plates is located at the front side of the transport carriage and extends at right angles to the direction of movement of the transport carriage. The transport carriage runs on two guide rails, and can be driven by a motor which is located on the upper side, this motor engaging, via a toothed wheel, a toothed rack drive arranged parallel to the direction of movement of the transport carriage. The transport carriage possesses, on the underside, a vacuum plate which is connected, via a number of holes, to a vacuum pump. On placing the vacuum plate of the transport carriage onto the upper side of the uppermost printing plate in the plate magazine, a pressure lower than atmospheric pressure is produced, whereby this printing plate is sucked against the vacuum plate. The transport carriage is thereafter driven, with the aid of the motor, toward the exposure stage where, as soon as it has reached its position above the exposure stage, it is lowered onto the stage and the vacuum is removed from the vacuum plate, so that the printing plate is released and is deposited on the exposure stage. The latter likewise contains a vacuum plate, to which a pressure lower than atmospheric pressure is applied, so that the printing plate is firmly pressed against the exposure stage during the exposure process.
In this known unit, that side of the printing plate which is coated with the photoconductor is touched by suction arrangements as the individual printing plate is lifted from the stack, and while these suction arrangements certainly enable the printing plate to be transported significantly more gently, compared to rollers or belts which are guided over the coated side of the printing plate in the case of other known units, the occurrence of mechanical or physical defects on the finished, developed printing plate, caused by the suction arrangements, cannot, however, be completely excluded.
A transport station for printing plates has been disclosed in U.S. Pat. No. 4,149,798.
This transport station possesses a stack region, in which a stock of printing plates is located, each printing plate being covered by a sheet of paper. A paper-dump facility is provided for the sheets of paper which are present between each two plates. The printing plates are transported from the stack region to a conveyor belt by means of a control mechanism which possesses a number of suction cups. The control mechanism is pivotably attached to a guide rod which is connected via gearing to a motor, so that this rod can execute a lateral sliding movement, forwardly and backwardly. Suction is applied to the uppermost printing plate from four or more suction cups of the control mechanism, after which the control mechanism pivots forwardly, and, at the same time, the guide rod is mechanically moved toward the conveyor belt. When the vacuum in the suction cups is removed, the control mechanism lowers itself and deposits the printing plate on the conveyor belt. A second control mechanism, above the sheet of paper, takes hold of the sheet, lifts it off and conveys it to the paper-dump facility.
The conveyor belt transports the printing plate onto an exposure stage which contains a number of holes in its surface and forms a chamber which is connected, via a suction line, to a vacuum pump. After the printing plate has been brought into the correct position on the surface of the exposure stage, it is held firmly on the latter by the generation of a vacuum. The exposure of the printing plate is effected with the aid of an exposure system.
The lifting of the uppermost printing plate from the stack is effected in the same manner as in the state of the art previously mentioned, wherein the photoconductive layer of the printing plate is touched by suction arrangements, and this process can lead to mechanical or physical defects on the finished printing plate.
In order not to mechanically touch the photoconductive layer of the printing plate during transport from a plate magazine to an exposure table, U.S. Ser. No. 250,124 (filed Apr. 2, 1981) proposes that a pivotable lifting cylinder be equipped with a piston rod which can be extended from, and be retracted into, the lifting cylinder, this piston rod carrying a dish-shaped plate for picking up, by suction, the uppermost printing plate of the stack, and for depositing the plate, and that the lifting cylinder be swung, about a rotation point, between a position at which the printing plate is received and a position at which it is deposited.
In U.S. Ser. No. 250,126 (filed Apr. 2, 1981) the object is achieved of transferring the printing plate, which has been lowered from above in a circular arc onto a supporting surface and of which the photoconductively coated side is facing upwardly, to a device for onward transport. This device is equipped with rollers; however the printing plate does not come to rest on the rollers as it is lowered, and the photoconductive layer of the printing plate does not come into contact with the rollers.
The proposed solution is that a pivotable lifting cylinder removes the printing form, which is configured as a printing plate, from a magazine and, executing a circular arc, deposits the printing form, by its front portion, onto a suction plate of the exposure table. Furthermore, a row of transport rollers and drive rollers is located, in each case, parallel to the two lateral edges of the printing plate, in the transport direction, and the transport rollers which are positioned first, in the transport direction of the printing plate, can be pivoted outwardly, out of the rows, in order to enable the front portion of the printing plates to be deposited, without jamming, onto the suction plate of the exposure table. After the printing plate has been deposited on the exposure table, the two mutually parallel roller rails with the transport rollers and drive rollers rest against the lateral edges of the printing plate and center the plate during the exposure. Since the rollers have a definite groove-depth, they overhang by this groove-depth at the edge or at the lateral edges of the printing plate, and project into its interior, thus making exposure of the printing plate all the way up to the edge impossible.